1. The History of Life on Earth
Chapter 25
The History of Life on Earth

2. Need to Know – The Big Six
The age of the Earth and when prokaryotic and eukaryotic life emerged.
Characteristics of the early planet and its atmosphere.
How Miller and Urey tested the Oparin-Haldane hypothesis and what they learned.
Methods used to date fossils and rocks.
Evidence for endosymbiosis.
How continental drift can explain the current distribution of species.

3. Past organisms were very different from those now alive
Overview: Lost Worlds
Past organisms were very different from those now alive
The fossil record shows macroevolutionary changes over large time scales including
The emergence of terrestrial vertebrates
The origin of photosynthesis
Long-term impacts of mass extinctions

4. Early Earth and the Origin of Life
Conditions on early Earth made the origin of life possible.
The current theory about how life arose indicates that chemical and physical processes on early Earth may have produced simple cells in a sequence of four main stages:
Small organic molecules were abiotically synthesized.
These molecules joined into macromolecules, such as proteins and nucleic acids.
All these molecules were packaged into protobionts, membrane-containing droplets, whose internal chemistry differed from that of the external environment.
Self-replicating molecules emerged that made inheritance possible.

5. The Formation of Earth
Earth was formed about 4.6 billions years ago, and life on Earth emerged about 3.8 to 3.9 billion years ago.
For the first three-quarters of Earth’s history, all of its living organisms were microscopic and primarily unicellular.
Earth’s early atmosphere likely contained water vapor and chemicals released by volcanic eruptions (nitrogen, nitrogen oxides, carbon dioxide, methane, ammonia, hydrogen, hydrogen sulfide).

6. Hypothetical Early Conditions of Earth
The hypothetical early conditions of Earth have been simulated in laboratories, and organic molecules have been produced.
A. I. Oparin and J. B. S. Haldane hypothesized that the early atmosphere, thick with water vapor, nitrogen, carbon dioxide methane, ammonia, hydrogen, and hydrogen sulfide, provided with energy from lightening and UV radiation, could have formed organic compounds, a primitive “soup” from which life arose.
Miller and Urey tested this hypothesis and produced a variety of amino acids.

7. The Miller-Urey experiment http://bcs. whfreeman

8. Protobionts Replication and metabolism are key properties of life.
Protobionts are aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure.
Protobionts exhibit simple reproduction and metabolism and maintain an internal chemical environment
Experiments demonstrate that protobionts could have formed spontaneously from abiotically produced organic compounds

9. Self-Replicating RNA and the Dawn of Natural Selection
The first genetic material was probably RNA, not DNA
RNA molecules called ribozymes have been found to catalyze many different reactions
For example, ribozymes can make complementary copies of short stretches of their own sequence or other short pieces of RNA
Early protobionts with self-replicating, catalytic RNA would have been more effective at using resources and would have increased in number through natural selection
The early genetic material might have formed an “RNA world”

10. The Fossil Record
The fossil record is the sequencd in which fossils appear in the layers of sedimentary rock that constitute Earth’s surface.
The fossil record reveals changes in the history of life on earth
Sedimentary rocks are deposited into layers called strata and are the richest source of fossils
Few individuals have fossilized, and even fewer have been discovered
The fossil record is biased in favor of species that
Existed for a long time
Were abundant and widespread
Had hard parts

11. How Rocks and Fossils Are Dated
Sedimentary strata reveal the relative ages of fossils:
In relative dating, the order of rock strata is used to determine the relative age of fossils.
The absolute ages of fossils can be determined by radiometric dating
Radiometiric dating uses the decay of radioactive isotopes to determine the age of the rocks or fossils.
It is based on the rate of decay, or half-life of the isotope (the time required for half the parent isotope to decay).

12. Key Events in Life’s History
Key events in life’s history include the origins of single-celled and multicelled organisms.
The earliest living organisms were prokaryotes.
About 2.7 billion years ago, oxygen began to accumulate in Earth’s atmosphere as a result of photosynthesis.
Eukaryotes apeared about 2.1 billion years ago.
Multicellular eukaryotes evolved about 1.2 billion years ago.
The colonization of land occurred about 500 million years ago, when plants, fungi, and animals began to appear on Earth.

13. Ceno- zoic Meso- zoic Humans Paleozoic Colonization of land Animals
Fig. 25-7
Ceno-
zoic
Meso-
zoic
Humans
Paleozoic
Colonization
of land
Animals
Origin of solar
system and
Earth 1 4
Proterozoic
Archaean
Prokaryotes
Key events in life’s history include the origins of single-celled and multicelled organisms and the colonization of land.
Billions of
years ago 2 3
Multicellular
eukaryotes
Single-celled
eukaryotes
Atmospheric
oxygen

14. The Effect of Oxygen on the Evolution of Life
By about 2.7 billion years ago, O2 began accumulating in the atmosphere and rusting iron-rich terrestrial rocks
A byproduct of oxygenic photosynthesis was the release of oxygen. As oxygen accumulated in the atmosphere:
It first dissolved into the surrounding water until the seas and lakes became saturated with oxygen.
Additional oxygen would then react with dissolved iron and precipitate as iron oxide.
Then additional oxygen finally began to “gas out” of the seas etc. and accumulate in the atmosphere.
The ozone layer was created.
As the ozone absorbed UV rays, the major source of energy for abiotic synthesis of organic molecules and primitive cells was terminated.

15. The Effect of Oxygen on the Evolution of Life
This “oxygen revolution” from 2.7 to 2.2 billion years ago
Posed a challenge for life
Provided opportunity to gain energy from light
Allowed organisms to exploit new ecosystems
The accumulation of oxygen had a tremendous impact on Earth:
Corrosive O2 attacks chemical bonds, and so doomed many prokaryotes.
Some survived in anaerobic environments (obligate anaerobe survivors).
Others adapted forms of cellular respiration.
Organic molecules and primitive cells were terminated.

16. Endosymbiosis and the First Eukaryotes http://highered. mcgraw-hill
The oldest fossils of eukaryotic cells date back 2.1 billion years
The hypothesis of endosymbiosis proposes that mitochondria and plastids (chloroplasts and related organelles) were formerly small prokaryotes living within larger host cells
An endosymbiont is a cell that lives within a host cell

17. Endosymbiosis and the First Eukaryotes
The prokaryotic ancestors of mitochondria and plastids probably gained entry to the host cell as undigested prey or internal parasites
In the process of becoming more interdependent, the host and endosymbionts would have become a single organism
Serial endosymbiosis supposes that mitochondria evolved before plastids through a sequence of endosymbiotic events

18. Endosymbiosis Theory (Lynn Margulis, 1970’s)
Model for origin of eukaryotes
(A) endomembrane system of eukaryotes may have evolved from specialized infoldings of plasma membrane of ancestral prokaryotes
(B) chloroplasts are descendants of photosynthetic prokaryotes, probably cyanobacteria
- proposed ancestors of mitochondria were endosymbiotic bacteria that were aerobic heterotrophs
a. may have first gained entry into larger cells as undigested prey or internal parasites
Be a good skeptic! Where’s the evidence?

19. Evidence Supporting the Endosymbiotic Theory
The endosymbiotic hypothesis proposes that mitochondria and plastids (chloroplasts) were formerly small prokaryotes that began living within larger cells. Evidence for this hypothesis includes:
Both organelles have enzymes and transport systems homologous to those found in the plasma membranes of living prokaryotes.
Both replicate by a splitting process similar to prokaryotes.
Both contain a single, circular DNA molecule, not associated with histone proteins.
Both have their own ribosomes which translate their DNA into proteins.

20. The Origin of Multicellularity
The evolution of eukaryotic cells allowed for a greater range of unicellular forms
A second wave of diversification occurred when multicellularity evolved and gave rise to algae, plants, fungi, and animals
Comparisons of DNA sequences date the common ancestor of multicellular eukaryotes to 1.5 billion years ago
The oldest known fossils of multicellular eukaryotes are of small algae that lived about 1.2 billion years ago

21. The Rise and Fall of Dominant Groups
The rise and fall of dominant groups reflect continental drift, mass extinctions, and adaptive radiations.

22. Consequences of Continental Drift
Formation of the supercontinent Pangaea about 250 million years ago had many effects
A reduction in shallow water habitat
A colder and drier climate inland
Changes in climate as continents moved toward and away from the poles
Changes in ocean circulation patterns leading to global cooling

23. Present Cenozoic 65.5 Millions of years ago 135 Mesozoic 251 Paleozoic
Fig
Present
Cenozoic
North America
Eurasia
65.5
Africa
South
America
India
Madagascar
Australia
Antarctica
Laurasia
135
Mesozoic
Millions of years ago
Gondwana
Figure The history of continental drift during the Phanerozoic eon
251
Pangaea
Paleozoic

24. Consequences of Continental Drift
The break-up of Pangaea lead to allopatric speciation
The current distribution of fossils reflects the movement of continental drift
For example, the similarity of fossils in parts of South America and Africa is consistent with the idea that these continents were formerly attached

25. Mass Extinctions
The fossil record shows that most species that have ever lived are now extinct
At times, the rate of extinction has increased dramatically and caused a mass extinction
In each of the five mass extinction events, more than 50% of Earth’s species became extinct

26. Consequences of Mass Extinctions
Mass extinction can alter ecological communities and the niches available to organisms
It can take from 5 to 100 million years for diversity to recover following a mass extinction
Mass extinction can pave the way for adaptive radiations

27. Adaptive Radiations
Adaptive radiation is the evolution of diversely adapted species from a common ancestor upon introduction to new environmental opportunities
Mammals underwent an adaptive radiation after the extinction of terrestrial dinosaurs
The disappearance of dinosaurs (except birds) allowed for the expansion of mammals in diversity and size
Other notable radiations include photosynthetic prokaryotes, large predators in the Cambrian, land plants, insects, and tetrapods

28. Developmental Genes & Morphological Changes
Major changes in body form can result from changes in the sequence and regulation of developmental genes.
Studying genetic mechanisms of change can provide insight into large-scale evolutionary change.
Genes that program development control the rate, timing, and spatial pattern of changes in an organism’s form as it develops into an adult.

29. Changes in Rate and Timing
Heterochrony is an evolutionary change in the rate or timing of developmental events…it can have a significant impact on body shape and thus contribute to the potential for evolutionary change:
The contrasting shapes of human and chimpanzee skulls are the result of small changes in relative growth rates
Heterochrony can alter the timing of reproductive development relative to the development of nonreproductive organs
In paedomorphosis, the rate of reproductive development accelerates compared with somatic development
The sexually mature species may retain body features that were juvenile structures in an ancestral species

30. Changes in Spatial Pattern
Substantial evolutionary change can also result from alterations in genes that control the placement and organization of body parts
Homeotic genes determine such basic features as where wings and legs will develop on a bird or how a flower’s parts are arranged
These are “master regulatory genes” that control organization!

31. Hox Genes
Hox genes are a class of homeotic genes that provide positional information during development
If Hox genes are expressed in the wrong location, body parts can be produced in the wrong location
For example, in crustaceans, a swimming appendage can be produced instead of a feeding appendage
Evolution of vertebrates from invertebrate animals was associated with alterations in Hox genes
Two duplications of Hox genes have occurred in the vertebrate lineage
These duplications may have been important in the evolution of new vertebrate characteristics

32. Hypothetical vertebrate ancestor (invertebrate)
Fig
Hypothetical vertebrate
ancestor (invertebrate)
with a single Hox cluster
First Hox
duplication
Hypothetical early
vertebrates (jawless)
with two Hox clusters
Second Hox
duplication
Figure Hox mutations and the origin of vertebrates
Vertebrates (with jaws)
with four Hox clusters

33. Evolution is NOT Goal Oriented
Evolution is like tinkering—it is a process in which new forms arise by the slight modification of existing forms
Most novel biological structures evolve in many stages from previously existing structures
Complex eyes have evolved from simple photosensitive cells independently many times
Exaptations are structures that evolve in one context but become co-opted for a different function
Natural selection can only improve a structure in the context of its current utility